The present invention relates to the field of ornamental plants.
The present invention is a new and distinct variety of Alnus maritima, commonly known as “Seaside Alder,” that is grown and utilized as an ornamental landscape tree or shrub. The new variety is known botanically as Alnus maritima subsp. oklahomensis and will be referred to hereinafter by the variety name, ‘September Sun.’
Plants of Alnus maritime are native to North America, and plants of subspecies oklahomensis specifically are native only to Oklahoma. The species is valued ornamentally for its dark green foliage, its bright-yellow, pendulous catkins that bloom in autumn, and its healthy appearance under conditions known to be stressful for other woody taxa. Subspecies oklahomensis is the most distinct of the three subspecies of Alnus maritima and represents the best of the species' ornamental and physiological traits. Efforts to develop superior cultivated varieties of subsp. oklahomensis began only recently in 1998.
Germplasm for ‘September Sun’ was obtained as seed from an open-pollinated parent growing naturally near Tishomingo, Okla. The inventors selected ‘September Sun’ as a single whole plant from a large seedling trial at Ames, Iowa in 2000. The inventors selected and confirmed ‘September Sun’ to be unique within subsp. oklahomensis for its faster growth, denser foliage, more symmetrical growth habit, and smaller, more slender infructescences shown in field trials with over 1000 plants. The vertical growth rate of ‘September Sun’ was shown to be approximately 73% greater than the mean for other genotypes of subsp. oklahomensis, and it volume growth rate greater than the mean. The strobili (cone-line infructescences) of ‘September Sun’ are 11% shorter and 18% smaller in diameter than the mean for other genotypes of subsp. oklahomensis. 
The inventors carried out the first asexual propagation of ‘September Sun’ in 2000 at Ames, Iowa by rooting softwood cuttings of the original single plant. All of the resulting plants exhibited the typical characteristics of ‘September Sun’. The inventors determined from successive generations of asexual propagation that the new variety ‘September Sun’ is stable and propagates true to type.
‘September Sun’ is registered with the International Cultivar Registration Authority, Brooklyn Botanical Garden (international authority for unassigned woody ornamentals), 1000 Washington Avenue, Brooklyn, N.Y. 11225-1099.